How can you render backfaces depending on whether the camera can see them?
Okay... I plugged in the camera data node's view distance socket and the light path node's ray length socket into a math node set to compare. The epsilon value is set to .2. I achieved an effect that is good enough, but still not perfect. The transmission, reflections, and light on the floor is correct, although the inside backfaces still cast a shadow+reflect teal light on the question mark. If anyone can help with that, it'd be perfect, but otherwise I'll probably close this question soon.